A common way of providing telephone subscribers with services that are not provided by the basic telephone station set is to incorporate them in an adjunct to the station set. One such adjunct is an automatic dialer. The adjunct dialer, which is typically connected in series between the associated telephone station set and the central office, often includes only the circuitry that is necessary to provide the dialing function. The adjunct dialer relies upon the associated telephone station set to perform the functions of going off-hook and acquiring dial tone from the central office. Thus when properly used, the adjunct dialer should be activated only after the telephone subscriber has removed the handset from the switchhook of the associated telephone station set and has recognized that dial tone has been returned from the central office.
A problem arises from this arrangement when the controls of the adjunct are operated before the handset is removed from the switchhook and dial tone has been returned. A path is completed across the telephone line shorting the line as automatic dialing begins. Thus if the adjunct is activated inadvertently such as may occur when a child plays with the controls of the adjunct, the telephone line is seized, and the central office will respond. No incoming calls may be received, and central office equipment is tied up. The adjunct does not release the path and open the line until the automatic dialing is completed.
One solution to this problem is known, namely to transmit switchhook status information over a separate lead. This lead proceeds from an extra contact of the associated telephone set's switchhook to the adjunct telephone station equipment. Off-hook or on-hook status information is provided over this lead to the adjunct equipment. Accordingly, the adjunct dialer is able to inhibit dialing if its controls are inadvertently operated when the telephone set is on-hook. There are then required in this embodiment three leads: the standard tip lead and ring lead, and an additional switchhook status lead. Hohmann et al., U.S. Pat. No. 3,128,351, which discloses an adjunct repertory dialer, is exemplary of apparatus which employ the additional lead for this purpose.
While this method provides a solution, it is desirable today, with the trend toward telephone station equipment modularity, to employ just two leads, the standard tip and ring leads, between the various items of station apparatus and to employ standard plug and jack arrangements to tie them together. In this manner, efficiencies in telephone equipment inventory control are achieved, and the subscriber is encouraged to install and to modify his telephone equipment arrangement without the aid of telephone company personnel. Exemplary plug and jack arrangements currently in use are described in Hardesty, U.S. Pat. No. 3,860,316, and its progeny and Krumreich et al., U.S. Pat. No. 3,850,497 and its progeny.
While it is desirable to provide a remote switchhook status check function without increasing the number of leads normally provided, it is likewise recognized that, in so doing, it is undesirable to increase the impedance of the telephone subscriber loop. Such an increase limits both the transmission and signaling distances from the central office where a telephone station set may be placed.